1. Field of the Invention
The present invention relates to a crystallization apparatus, a crystallization method, and a phase shift mask and a filter for use in these apparatus and method. More particularly, the present invention relates to an apparatus and a method which irradiate a polycrystal semiconductor film or layer, or an amorphous semiconductor film or layer with a laser beam phase-modulated by using a phase shift mask, thereby generating a crystallized semiconductor film.
2. Description of the Related Art
Materials of a thin film transistor (TFT) used in a switching element or the like which controls a voltage to be applied to pixels of, e.g., a liquid crystal display (LCD) are conventionally roughly classified into amorphous silicone and polysilicon.
The polysilicon has a higher electron mobility than that of the amorphous silicon. Therefore, when a transistor is formed by using the polysilicon, a switching speed is higher than that obtained when using the amorphous silicon, and a response speed of a display using such a transistor is also higher. Further, a peripheral LSI can be constituted by such a thin film transistor. Furthermore, there is an advantage that a design margin of any other component can be reduced. Moreover, when peripheral circuits such as a driver circuit or a DAC as well as a display main body are incorporated into a display, these peripheral circuits can be operated at a higher speed.
The polycrystal silicon is composed of a group of crystal grains, and has a lower electron mobility than that of monocrystal silicon. In a small transistor formed by using the polycrystal silicon, irregularities in a crystal grain boundary number in a channel portion become a problem. Thus, in recent years, in order to increase the electron mobility and reduce irregularities in the crystal grain boundary number in the channel portion, there has been proposed a crystallization method which generates monocrystal silicon with a large particle size.
As this type of crystallization method, there has been conventionally known “phase control ELA (Excimer Laser Annealing)” which irradiates a phase shift mask which is in close vicinity to a polycrystal semiconductor film or an amorphous semiconductor film in parallel, with an excimer laser beam, thereby generating a crystallized semiconductor film. The detail of the phase control ELA is disclosed in, e.g., “Surface Science Vol. 21, No. 5, pp. 278–287, 2000”.
In the phase control ELA, a light intensity distribution having an inverse peak pattern that a light intensity is minimum or substantially zero at a point corresponding to a phase shift portion of a phase shift mask (pattern that a light intensity is substantially zero at the center and the light intensity is suddenly increased toward the circumference) is generated. A polycrystal semiconductor film or an amorphous semiconductor film is irradiated with the light having this light intensity distribution with the inverse peak pattern and thus locally fused. As a result, a temperature gradient is generated in a fusion area or areas of the film in accordance with the light intensity distribution, a crystal nucleus or nuclei are formed at parts which first solidify in accordance with points where the light intensity are substantially zero, and crystals grow in a lateral direction (lateral growth) from the crystal nuclei toward the circumference, thereby generating monocrystal grains with a large particle size.
The phase shift mask generally used in a prior art is a so-called line type phase shift mask, which is constituted by pairs of rectangular areas which are alternately repeated along one direction, and a phase difference of π (180 degrees) is given between the adjacent areas. In this case, since a boundary between the adjacent areas constitutes a phase shift portion, light passing through the mask has such a light intensity distribution having an inverse peak pattern such as that a light intensity is minimum or substantially zero at a position on a line corresponding to the phase shift portion and the light intensity is one-dimensionally increased toward the circumference. Thus, a polycrystal semiconductor film or an amorphous semiconductor film is irradiated with such a light.
As described above, in the prior art using the line type phase shift mask, a temperature becomes lowest along the line corresponding to the phase shift portion, and a temperature gradient is generated along a direction orthogonal to the line corresponding to the phase shift portion. That is, a crystal nucleus or nuclei are generated on the line corresponding to the phase shift portion, and crystallization advances from each crystal nucleus along a direction orthogonal to the line corresponding to the phase shift portion. In the prior art using such a phase shift mask, it is general that the light intensity distribution in an intermediate portion between two adjacent inverse peak pattern portions has an irregular undulation (which will be described later in detail with reference to FIG. 5). In this case, a crystal nucleus may be possibly generated at a position where the light intensity is low (i.e., at an undesired position) in the undulation of the intermediate portion in a crystallization process. Moreover, the lateral growth which has begun from the crystal nucleus toward the circumference may stop at a part in the intermediate portion where the light intensity is decreased, which obstructs growth of a large crystal.